Vehicles with an internal combustion engine may be fitted with fuel vapor recovery systems (vapor storage systems) wherein vaporized hydrocarbons (HCs) released from a fuel tank are captured and stored in a fuel vapor canister containing a quantity of fuel-absorbing material such as activated charcoal. Eventually, the fuel vapor canister may become filled with an amount of fuel vapor. The fuel canister may be cleared of fuel vapor by way of a purging operation. A fuel vapor purging operation may include opening a purge valve to introduce the fuel vapor into the cylinder(s) of the internal combustion engine for combustion so that fuel economy may be maintained and fuel vapor emissions may be reduced.
Activated charcoal has been found to be a suitable fuel vapor adsorbing material to be used in such a canister device because of its extremely porous structure and very large surface area to weight ratio. However, this porous structure can lose some of its adsorption efficiency when coated with liquid fuel. This may occur if, for example, during refueling a pump operator adds fuel after an initial automatic shut-off. For instance, in an attempt to maximize the amount of fuel pumped into the tank, a pump operator may dispense additional fuel in what is commonly referred to as “trickle-filling”, If liquid has entered the fuel vapor recovery lines (evap recovery lines) and a purge cycle is commanded at the next engine start, the liquid can get sucked into the canister and corrupt the activated carbon. This may lead to increased HC emissions. Additionally, if liquid fuel in the canister or purge line is purged to the intake, a reduction of engine power may result from an extremely low air-fuel ratio (A/F). Further, HC emissions from the engine exhaust may increase from the low air-fuel ratio (A/F). Accordingly, it is desired to easily diagnose and mitigate the presence of liquid fuel in the evap recovery lines.
Toward this end, US Patent Application US 2007/0131204 A1 teaches a method of detecting whether liquefied fuel exists in a canister purge line based on a fuel level in a fuel tank higher than a pre-set level. If the fuel level is greater than a pre-set level, an air ratio is measured by an oxygen sensor in the exhaust manifold. The purge control valve is then opened for a pre-set time period, the air ratio is measured again, and a difference is calculated between the two. If the difference is less than or equal to a first value, and the air-fuel ratio after opening the purge control valve is less than or equal to a second value, then it is deemed that liquefied fuel exists in the canister purge line. If liquefied fuel is deemed to exist in the canister purge line, the purge control valve may be closed for a pre-set time period. Thus, loss of engine power due to a low A/F caused by liquefied fuel in the canister purge line is prevented by detecting liquefied fuel in the canister purge line in advance. However, the inventors herein have recognized potential issues with such a method. For example, the method is such that detection of fuel in the canister purge line does not alleviate or prevent the possibility of vapor canister adsorption degradation due to liquid in the vapor canister. Further, the method does not provide mitigating actions to purge liquid fuel from the evap recovery lines. An attractive alternative therefore, is a method that includes both detection and mitigation of fuel carryover in the evap line(s), such that liquid fuel does not come into contact with the activated charcoal housed within the vapor canister.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, the issues described above may be addressed by a method for a vehicle including, during refueling a tank which supplies fuel to a combustion engine, venting the tank to atmosphere through a vapor storage system, and after refueling and when pressure decay in the tank is less than a threshold, sealing the vapor storage system from atmosphere and running the engine un-fueled in reverse to force air from an intake manifold of the engine through the vapor storage system into the tank to return liquid fuel in the evap recovery lines to the fuel tank.
As one example, pressure decay in the tank after refueling may comprise a first pressure decay rate, and pressure decay in the tank while running the engine in reverse may comprise a second pressure decay rate, wherein the engine may be continued to run in reverse responsive to the second pressure decay rate greater than another threshold until pressure in the tank decreases to atmospheric pressure. In this way, the presence of liquid fuel in the evap recovery lines may be quickly diagnosed, and mitigating action may be taken to return the fuel to the tank.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.